Screenable contact structure and method for semiconductor devices

ABSTRACT

An ink composition for deposition upon the surface of a semiconductor device to provide a contact area for connection to external circuitry is disclosed, the composition comprising an ink system containing a metal powder, a binder and vehicle, a metal frit, and a fluxing agent. The ink is screened onto the semiconductor surface in the desired pattern and is heated to a temperature sufficient to cause the metal frit to become liquid. The metal frit dissolves some of the metal powder and densifies the structure by transporting the dissolved metal powder in a liquid sintering process. A two-step heating process may be used, the first step serving to activate the fluxing agent to prepare the semiconductor surface. The sintering process may be carried out in various types of atmospheres depending upon the metal powder utilized. A small amount of dopant, semiconductor material or a semiconductor-dopant eutectic alloy powder may be added to the ink system.

This application is a continuation-in-part of application Ser. No.913,872, filed June 8, 1978 now U.S. Pat. No. 4,219,448.

BACKGROUND OF THE INVENTION

The present invention relates to structures and methods for formingelectrical contact areas on semiconductor devices, whereby such devicesmay be connected to external circuitry. More particularly, the presentinvention relates to the formation of such contact areas by screening ametal ink onto the surface of a semiconductor device and subjecting theink to a firing process in a manner similar to that utilized in thickfilm technology.

Numerous methods are utilized for metallizing and electricallycontacting semiconductor devices. Such methods, used singly or incombination, include thermal compression bonding, vapor deposition,plasma sputtering, electrolytic plating, and electroless plating. Thesemethods often require multiple, repetitive processing steps and the useof quite expensive materials such as gold, palladium, rhodium, platinumand silver. The plating methods raise problems of masking difficulty andpoor adhesion as well as front layer metallurgical punch-through insintering, poor conductivity, variable solderability, and poor moisturestability. The vapor deposition processes often result in a moisturesensitive contact that can degrade as a function of time in normalambients. The sputtering processes have been difficult to make costcompetitive, especially when sintering is required between depositions.

Thick film technology originated in the ceramics industry, whereinprecious metal decorative inks such as platinum or gold were screenedonto ceramic items and fired to achieve permanence. Thick films aredefined as being within the range of 12 μm (0.0005 inches) to 50 μm(0.002 inches). Thick film technology was introduced into electronics toprovide metallization on a ceramic substrate, and later was used tofabricate multilayer chip capacitors; however, it has primarily beendirected to deposition upon metal oxide (i.e., ceramic) surfaces ratherthan upon surfaces that are to be kept relatively free of oxides.

Recently thick film technology has been utilized to create contact areason semiconductor devices, particularly large devices such as solarcells. Both a grid to act as a front contact and a back contact havebeen fabricated using conventional thick film techniques. Thesetechniques utilize a metal ink system consisting of a metal powder, abinder and vehicle, and a metal oxide (glass) frit. Typically the metalpowder has been silver, sometimes with a small amount of palladiumadded, and the glass frit has typically been a lead oxide, a bismuthoxide, or a borosilicate. Typical binders and vehicles have been ethylcellulose and butyl carbitol, respectively; the binder gives the inkscohesion and the vehicle provides the necessary viscosity prior tofiring. The ink is screened onto the semiconductor device in the desiredpattern by conventional screening techniques, and is then fired at atemperature sufficient to cause the glass frit to become liquid. In thefiring process, a portion of the metal powder is dissolved in the liquidfrit, and the dissolved metal is then transported to areas of highsurface energy such as the negative curvature between undissolved metalparticles in contact. This effect causes grain growth and layershrinkage, resulting in a coherent metal structure wherein the glassfrit fills the interstices between cemented metal powder grains.

The above process is typically carried out in an oxidizing atmosphere toremove the binder by oxidation, and at a temperature of 400° C. to 900°C. depending on the melting point of the glass frit. Because it iseasily oxidizable, an inexpensive metal such as copper cannot be easilyused in such a process, and the surface of the semiconductor device alsotends to oxidize and thus create an electrically insulating layer. Ifthe process departs from optimum firing conditions, the glass frit islikely to segregate and migrate to the free surface, making the contactunsolderable, or to the semiconductor surface, increasing the seriesresistance. Efforts to improve either situation by a hydrofluoric aciddip sometimes cause the contact to fall off the semiconductor surface.Also, such contacts often lack adhesion and cohesiveness, and tend toleach off easily in conventional solders because of the high solubilityof silver in tin. Additionally, it is important, particularly in thecase of solar cells, that the contact withstand environmental stressessuch as moisture, humidity, thermal cycling, vibration, andcontamination. Some of the glass frit systems have demonstrated poorresistance to such environmental stresses, particularly moisture.

A metal frit system has been used as the liquid phase medium in asintering process in the field of powder metal technology for theproduction of cemented carbides such as tungsten carbide cutting andgrinding tools. In this process a metal powder such as tungsten carbidepowder is mixed with about six percent by weight of cobalt powder inaddition to vehicle and binder. The mixture is then pressed to shape,prefired in hydrogen to remove the binder, and liquid sintered in avacuum at approximately 1400° C. The cobalt powder, whose melting pointis depressed by the dissolved carbon of the tungsten carbide, furnishesthe liquid medium. Such use of a metal frit, however, has heretoforebeen unknown in the field of thick film technology.

SUMMARY OF THE INVENTION

According to the present invention there are provided a metal inkcomposition and method for fabricating electrical contacts on thesurface of a semiconductor device such as a solar cell. The compositioncomprises screenable conductive metal powder, a binder, a vehicle, ametal frit preferably having a relatively low melting point, and afluxing agent. A small amount of an appropriate semiconductor powder maybe added to the system to prevent punch-through, as may a small amountof an appropriate dopant or of a semiconductor-dopant eutectic alloypowder to enhance the electrical junction characteristics or to improvethe conductivity of the device. The method for applying the metal inkcomposition follows generally the conventional screening and firingmethods of thick film technology, except that the firing may take placeat a considerably lower temperature, the precise temperature dependingPG,6 on the metal chosen as the liquid frit medium, and the firing maybe performed in other than an oxidizing atmosphere. A two-step firingprocess may be utilized, the first step serving to activate the fluxingagent to achieve an oxide-free surface and the second step serving asthe liquid sintering step.

Improved adherence and cohesiveness are achieved, and better electricalcharacteristics result from the absence of insulating material in theink and the reduced tendency to form oxides either on the semiconductorsurface or the metal powder. Less expensive metals may therefore beutilized, and the ability of the contact to withstand environmentalstresses is improved. The latter effect is believed possibly to be dueto the formation of metal silicides in the sintering process, which tendto create very stable interfaces.

Accordingly, it is an object of the present invention to provide a metalink composition for deposition upon a substrate, wherein the liquidphase medium is a metal frit.

It is another object of the present invention to provide a reproducible,reliable contact to semiconductor surfaces that can be fabricated on alarge area device and yet remain low in cost.

It is a further object of the present invention to provide such acontact having improved electrical characteristics and ability towithstand environmental stresses.

It is another object of the present invention to provide an improvedmethod for sintering a metal ink structure to a semiconductor surface.

It is a still further object of the present invention to provide such amethod wherein the sintering may be performed at a lower temperaturethan heretofore and in any desired atmosphere.

These and further objects and advantages of the present invention willbecome apparent from the following detailed description of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention a metal ink composition forfabricating electrical contacts upon semiconductor devices comprises aconductive metal powder, a binder and vehicle, and a metal frit as aliquid phase medium. The term "metal" in the present context will beunderstood to include elemental metals as well as alloys and/orcombinations thereof, although this may be explicitly set forthhereinafter as well. The metal frit has a relatively low melting point,preferably between 200° and 750° C., and preferably comprises a powderof lead, tin, zinc, or cadmium, and in certain cases, to be discussed ingreater detail hereinafter, indium, bismuth, antimony or aluminum and/oralloys or combinations thereof. If a chemically reducing metal isutilized for the metal frit, a better ohmic contact may be achieved bythe tendency of the frit to dissolve native oxide on the semiconductorsurface. The metal frit preferably constitutes one to twenty percent ofthe ink composition by weight.

Because the above metals have a considerably lower melting point thanmost glass frits previously utilized, lower firing temperatures arepossible. For this reason an easily oxidizable base metal may be used asthe metal powder of the system particularly when sintering is performedin other than an oxidizing atmosphere. Because it is relativelyinexpensive and highly conductive, copper would normally be preferred asthe metal powder, although other conductive metals and/or alloys orcombinations thereof may be utilized. To achieve a sufficiently lowcontact resistance to the semiconductor device, the metal chosen shouldhave an electrical resistivity at 0° C. of not more than approximately7.0 ohm-cm. Depending upon the metal powder chosen, firing may beperformed in an oxidizing atmosphere such as air, a neutral atmospheresuch as nitrogen, or a reducing atmosphere such as hydrogen or forminggas. Optimum results have been achieved thus far by sintering inhydrogen an ink containing copper as the metal powder, as will bedescribed in greater detail hereinafter.

The binder will preferably be of a type that may be used in either anoxidizing or a reducing atmosphere and that becomes fugitive at arelatively low temperature, such as an acrylic polymer or polyvinylalcohol (PVA). The vehicle may be any solvent for the particular binder,such as water and ethanol or methanol combined with a small quantity oftriethanolamine (TEA) when PVA is the binder or any well-known solventfor acrylic polymers. The binder and vehicle together may comprisefifteen to thirty percent by weight of the total ink structure. It willbe apparent that the vehicle should be chosen to be incompatible withany vehicle-binder system that may be present on the substrate, althoughno such system is normally present in the case of a semiconductor devicesuch as a solar cell. The screen mask emulsion also must be chosen to beincompatible with the ink vehicle.

It has been found desirable under most circumstances to add a very smallamount, such as 1% to 10% by weight, of a fluxing agent to the inksystem, in a manner similar to conventional techniques used in ceramicsand metallurgy. In the context of the present invention, however, theterm "fluxing agent" is to be understood as referring not only tosubstances which remain a part of a final structure, such as rosin inthe case of a solder flux, but also to substances which may decomposeand be at least partially removed from the ink during firing. Withrespect to the structure of the present invention, such a fluxing agentis preferably of a type tending to dissociate oxides such as silicondioxide to help insure better electrical and mechanical contact to thesemiconductor device. Silver fluoride is presently preferred for thispurpose, although lithium fluoride, cesium fluoride, Teflon, ammoniumfluoride, and other substances may also be utilized. When silverfluoride is used as the fluxing agent, for example, and the firingtemperature is at least 435° C., the silver fluoride decomposes,releasing nascent fluorine which dissociates silicon dioxide and formsgaseous silicon tetrafluoride, releasing free oxygen and leaving behindmetallic silver which becomes a part of the contact structure. Thus anynative or thermal oxide on the semiconductor surface is removed,resulting in the formation of an intimate, stable contact between thesemiconductor and the metal powder and resulting in improved electricaland mechanical characteristics.

Optionally, a small amount of a metal such as indium or thallium may beadded to the ink to improve wetting of the liquid metal frit to themetal powder grains. Also, it may be desirable to include a small amountof semiconductor powder of the same type from which the device isfabricated, such as silicon in the case of a silicon device, to the inksystem to prevent metallurgical punch-through of a very thinelectrically active layer. Particularly in the case of solar cells, thislayer may be on the order of 0.5 μm or less in thickness, and the priorprocesses described above involve the risk of partially or completelydissolving this thin layer into the metal ink during the sinteringprocess. This may be avoided by saturating the ink system with thesemiconductor powder described above, typically one to two percent ofthe total ink system by weight, which is believed to help preventfurther uptake of semiconductor material from the surface layer.

As mentioned above, in certain cases it may be desirable to utilize suchmetals as indium, bismuth, antimony or aluminum as the metal frit in astructure according to the present invention. Such materials areparticularly useful when it is desired to dope the semiconductor surfacelayer to enhance N or P type conduction and achieve minimum seriesresistance, and also can provide a back surface field. Alternatively,such a dopant could be added in small quantities to an ink systemcontaining another metal as the liquid phase medium.

It has also been found that addition of a small amount, such as 5% byweight, of a powder comprising a semiconductor-dopant eutectic alloyenhances the electrical characteristics of the resulting contact byproviding an epitaxially deposited heavily doped semiconductor surfaceunder the contact. The powder may be, for example, an aluminum-siliconeutectic alloy or a germanium-aluminum eutectic alloy. The powder shouldbe prepared prior to addition to the ink system by alloying theconstituent metals such as in hydrogen and reducing the resulting alloyto a powder. Because germanium has a lower melting point and a lowerenergy gap than silicon, the germanium-aluminum alloy has been foundpreferable in processes utilizing lower firing temperatures.

Application of the metal ink structure to the semiconductor device isperformed by conventional screening techniques as set forth above,preferably utilizing a stainless steel wire mesh screen coated with anemulsion mask. An electrical junction will have previously been formedin the device by any conventional process, and the surface of the devicewill have been cleaned. In the case of solar cells, the ink typically isdeposited in a grid pattern on the front of the cell with the lines ofthe grid spaced apart on the order of 0.5 to 1.0 cm (0.2 to 0.4 inches)and being on the order of 0.2 mm (0.08 inches) in width, the gridfurther being interconnected by a collecting bar on the edge or in thecenter of the device for connection to external circuitry byconventional techniques such as soldering, bonding or the like. The inkgenerally is deposited on the entire back of the device, sometimesleaving a thin ring around the edge. The front and back inks generallyare not fired at the same time; rather, one side generally is inked andfired, and the other side is then inked and fired. Simultaneous firingmay be performed, however. It will be understood that differently dopedinks, if doped inks are employed, may be used on the front and the backof the device, depending upon the electrical characteristics of thesemiconductor material on each side.

After the ink is screened onto the semiconductor surface, it is allowedto dry and the device is then fired. surface, it is allowed to dry andthe device is then fired. The firing for most typical ink structuresaccording to the present invention may be performed at a temperature ofbetween approximately 200° C. and 750° C. and typically for ten tofifteen minutes, although the optimum temperature and time will varywith the particular constituents of the ink composition and with theamount thereof present in the composition. If copper is used as themetal powder, for example, a firing temperature of approximately 550° C.has been found acceptable. At this temperature, it is best to fire sucha composition in a reducing atmosphere such as hydrogen. Otherinexpensive metals that may be utilized include nickel and aluminum. Ofcourse, noble metals such as silver or gold, or any other conductivemetal, may be utilized although the costs thereof are greater. Dependingupon the particular metal, the firing may be performed in air ifexcessive oxidation of the metal does not occur.

During firing, as explained above, a liquid sintering process takesplace whereby it is believed that the metal powder, which is dissolvedin the liquid frit to beyond its saturation point, redeposits on areasof high surface energy such as the negative curvature between twoundissolved metal particles in contact. This causes grain growth andlayer shrinkage, resulting in a coherent metal structure in which themetal frit is dissolved to the limit of its solubility in the regrownportions of the metal grains. If the device is overfired, the majoreffect will be to cause a larger fraction of the metal frit to bedissolved in this fashion. As opposed to the result when glass frit inksare utilized, this effect will not cause significant changes in theelectrical characteristics of the contact because the metal frit isconductive. It is, of course, desired that the sintered structurewithstand soldering, and it has been shown that the grain structures aresufficiently well interlocked and the amount of interstitial metal fritsufficiently small in volume that contact erosion by soldering isinsignificant.

A two-step firing process has been found desirable under somecircumstances and is presently preferred for insuring the removal of theoxide from the semiconductor surface when the ink contains a fluxingagent and when a base metal such as copper is used as the metal powder.The first step is to decompose the fluxing agent in order to activate itso that it completely dissociates any surface oxide, as previouslydescribed, thus establishing a metallurgical bond between the surfaceand the metal powder. This step is preferably performed in a neutralatmosphere such as nitrogen when copper is the metal powder and at atemperature of from 435° C. to 750° C. when silver fluoride is thefluxing agent for 1 to 10 minutes, or just long enough for the ink toreach the furnace temperature. The second step is then to sinter thecomposition at the same or a higher temperature and preferably in areducing atmosphere for 2 to 20 minutes. The second step may, of course,be performed in the same furnace by readjusting the temperature andatmosphere as appropriate, or in a separate or a multi-section furnace.

If the contact is to be formed on a solar cell, which normally carriesan anti-reflective coating in its completed form to increase energyabsorption, the ink may be deposited and fired either before theanti-reflective coating is deposited or, if the ink contains anappropriate fluxing agent, afterwards. It has been found that a fluxingagent such as silver fluoride will dissolve the anti-reflective layer inthe areas where the ink is deposited over such a layer, particularlywhen the two-step firing process described above is utilized. Thus, itmay be desirable to screen the ink onto the cell after deposition of theanti-reflective layer, to eliminate the need to form contact windows inthe anti-reflective layer to make electrical contact with the gridpattern.

While standard, commercially available metal powders may be mixedtogether and used as the metal powder and the metal frit in the presentinvention, it has been found that electroless plating of the metal fritonto the metal powder grains results in more uniform distribution of thefrit metal in the ink, improving wetting and inhibiting oxidation of themetal powder grains during firing.

It will be apparent that there have been provided by the presentinvention a new and useful screenable contact structure and method forfabricating such a structure, the resulting contact having improvedqualities of adherence, cohesiveness and resistance to environmentalstresses. While presently preferred embodiments of the structure andmethod have been described, many variations and modifications thereofwill be apparent to those skilled in the art, and it is intended toinclude all such variations and modifications within the scope of theappended claims.

I claim:
 1. A method of fabricating an electrical contact upon asubstrate, comprising the steps ofscreening a metal frit ink comprisinga metal powder and a metal frit in a predetermined pattern upon saidsubstrate, said ink containing a fluxing agent capable of dissolvinginsulating substances on said substrate, dissolving any insulatingsubstances present on said substrate by activating said fluxing agent,and sintering said ink by firing to a temperature above the meltingpoint of the metal frit and below the melting point of the metal powderto create a coherent metal contact.
 2. The method of claim 1 whereinsaid fluxing agent is a metal fluoride and said step of dissolvingcomprises heating said ink to a temperature sufficient to decompose saidfluoride.
 3. A metal powder composition for deposition and firing upon asubstrate, comprisinga screenable metal powder, a binder, a vehicle, ametal frit for use as a liquid medium to transport the metal powder in aliquid sintering process and thereby create a coherent metal structure,and a fluxing agent comprising silver fluoride for dissolving insulatingsubstances from said substrate.
 4. A metal powder composition fordeposition and firing upon a substrate, comprisinga screenable powder ofa metal having an electrical resistivity of 7.0 ohm-cm or less at 0° C.,a binder, a vehicle, a metal frit, and a fluxing agent comprising silverfluoride for dissolving insulating substances from said substrate.
 5. Ametal powder composition for deposition and firing upon a substrate,comprisinga screenable powder of a metal having an electricalresistivity of 7.0 ohm-cm or less at 0° C., a binder, a vehicle, a metalfrit having a melting point approximately in the range of 200° C. to750° C., and a fluxing agent comprising silver fluoride for dissolvinginsulating substances from said substrate.
 6. A composition according toclaim 3, 4, or 5 further including a semiconductor powder.
 7. Acomposition according to claim 3, 4, or 5 further including a dopantselected from the group consisting of N and P type dopants.
 8. Acomposition according to claim 3, 4, or 5 wherein the amount of themetal frit is in the range of one to twenty percent by weight of thecomposition.
 9. A composition according to claim 3, 4, or 5 wherein themetal powder is selected from the group consisting of copper, nickel,aluminum, silver, and gold, and alloys and combinations thereof.
 10. Acomposition according to claim 3, 4, or 5 wherein the metal frit isselected from the group consisting of lead, tin, zinc, cadmium, indium,bismuth, antimony, aluminum, and alloys and combinations thereof.
 11. Ametal powder composition for deposition and firing upon a substrate,comprisinga screenable metal powder selected from the group consistingof copper, nickel, aluminum, silver, gold, and combinations and alloysthereof, a binder, a vehicle, a metal frit selected from the groupconsisting of lead, tin, zinc, cadmium, indium, bismuth, antimony,aluminum, and combinations and alloys thereof, a semiconductor powder, adopant selected from the group consisting of N and P type dopants, and afluxing agent comprising silver fluoride for dissolving insulatingsubstances from said substrate.
 12. A metal powder composition fordeposition and firing upon a semiconductor substrate, comprisingascreenable metal powder, a binder, a vehicle, a metal frit for use as aliquid medium to transport the metal powder in a liquid sinteringprocess and thereby create a coherent metal structure, and a powdercomprising a semiconductor-dopant eutectic alloy.
 13. A metal powdercomposition for deposition and firing upon a semiconductor substrate,comprisinga screenable powder of a metal having an electricalresistivity of 7.0 ohm-cm or less at 0° C., a binder, a vehicle, a metalfrit, and a powder comprising a semiconductor-dopant eutectic alloy. 14.A metal powder composition for deposition and firing upon asemiconductor substrate, comprisinga screenable powder of a metal havingan electrical resistivity of 7.0 ohm-cm or less at 0° C., a binder, avehicle, a metal frit having a melting point approximately in the rangeof 200° C. to 750° C., and a powder comprising a semiconductor-dopanteutectic alloy.
 15. A composition according to claim 12, 13, or 14further including a semiconductor powder.
 16. A composition according toclaim 12, 13, or 14 further including a dopant selected from the groupconsisting of N and P type dopants.
 17. A composition according to claim12, 13, or 14 further comprising a fluxing agent.
 18. A compositionaccording to claim 12, 13, or 14 wherein the amount of the metal frit isin the range of one to twenty percent by weight of the composition. 19.A composition according to claim 12, 13, or 14 wherein the metal powderis selected from the group consisting of copper, nickel aluminum,silver, and gold, and alloys and combinations thereof.
 20. A compositionaccording to claim 12, 13, or 14 wherein the metal frit is selected fromthe group consisting of lead, tin, zinc, cadmium, indium, bismuth,antimony, aluminum, and alloys and combinations thereof.
 21. A metalpowder composition for deposition and firing upon a substrate,comprisinga screenable metal powder selected from the group consistingof copper, nickel, aluminum, silver, gold, and combinations and alloysthereof, a binder, a vehicle, a metal frit selected from the groupconsisting of lead, tin, zinc, cadmium, indium, bismuth, antimony,aluminum, and combinations and alloys thereof, a semiconductor powder, adopant selected from the group consisting of N and P type dopants, afluxing agent, and a powder comprising a semiconductor-dopant eutecticalloy.
 22. A composition according to claim 12, 13, or 14 furthercomprising a fluxing agent selected from the group consisting offluorides.
 23. A composition according to claim 12, 13, or 14 furthercomprising a fluxing agent comprising silver fluoride.
 24. A compositionaccording to claim 21 wherein the fluxing agent is selected from thegroup consisting of fluorides.
 25. A composition according to claim 21wherein the fluxing agent comprises silver fluoride.